1. Field of the Invention
This invention relates to a process for producing a crystal article.
The present invention can be suitably used for, for example, SOI technique.
2. Related Background Art
In the prior art, the single crystalline thin film to be used for semiconductor electronic device, photoelectric device, etc. has been formed by epitaxial growth on a single crystalline substrate. However, for epitaxial growth of a single crystalline thin film on a single crystalline substrate, it is necessary to take matching in lattice constant and coefficient of thermal expansion between the single crystalline material of the substrate and the epitaxial growth layer, and there has been involved the problem that the kinds of the substrate material were limited to extremely narrow scope for formation of a single crystalline layer capable of preparing a device of good quality.
On the other hand, in recent years, research and development have been actively done about three-dimensional integrated circuits for accomplishing high integration and multi-functionality by forming semiconductors by lamination in the direction normal to the substrate, and also research and development about a large area semiconductor device such as a solar battery in which elements are arranged in an array on an inexpensive glass or switching transistor of liquid crystal picture elements, etc. are becoming more active year by year.
What is common in these researches and developments, the technique to form a semiconductor film on an amorphous insulating material and form an electronic element such as transistor, etc. thereon is required. Among them, it has been particularly desired to have a technique to form a single crystalline semiconductor of high quality on an amorphous insulating material.
However, generally speaking, when a thin film is formed on an amorphous insulating substrate such as SiO.sub.2, etc., due to deficiency of long length order of the substrate material, the crystal structure of the deposited film becomes amorphous or polycrystalline, whereby it was very difficult to form a single crystalline semiconductor of high quality. Here, amorphous film refers to one with the state in which short length order to the minimum extent on the order of atom may be maintained, but there is no longer length order, while polycrystalline film refers to one in which single crystal grains having no specific crystal direction are gathered as separated with grain boundaries.
As a solution of the problems as mentioned above in the prior art, European Patent Application EP 244081 A1 proposes a method for forming a crystal by permitting a single crystal to grow on the single nucleus as the growing center, on the nucleation surface (S.sub.NDL) provided on a substrate surface which is constituted of a material sufficiently larger in nucleation density (ND) than the material constituting said substrate surface and has an area sufficiently small to the extent that only a single crystal can be grown, and by use of this method, it is shown possible to form a single crystal also on an insulating amorphous substrate.
In the invention described in the above European Patent Application, when the material of different kind constituting the nucleation surface (S.sub.NDL) is smaller in etching speed than the material constituting the substrate surface, it is difficult to effect etching selectively and form the nucleation surface (S.sub.NDL) with good precision, and therefore improvement in this respect has been desired.